Method for analyzing the joinder between a pair of abutting members

ABSTRACT

A transducer induces vibrations into a member including at least a pair of members joined to one another along a common surface. An exposed section of the two members, which may or may not include one line of their joinder, is illuminated with a source of coherent light while it is being vibrated and a photographic plate is exposed to both the reflected light and the direct coherent light. The plate is developed to provide a hologram which is then properly illuminated to recreate a visual image of the surface section. The visual image will contain fringe lines and/or shaded areas as a result of interference between light waves reflected from the member at the high and low points of its vibration. Anomalies in these lines and areas will reveal discontinuities in the joinder between the two members which affect the pattern of vibration of the surface section.

tes l tent mated Grant Feb. 29, 1972 Inventor: Ralph M. Grant, AnnArbor, Mich.

[73] Assignee: G. C. Optronics Inc., Ann Arbor, Mich.

[221 Filed: Mar. 18, 1968 [21] App], No.: 745,054

Related US. Application Data [63] Continuation-in-part of Ser. No.633,068, Apr. 24,

1967, abandoned.

[52] US. Cl ..73/67, 73/67.2, 73/71.3, 350/35 [51] llnt. Cl. ..G01n29/04 [58] Field of Search ..73/67, 67.2, 67.5 H, 71.3; 350/35 [56]References Cited UNITED STATES PATENTS 2,645,933 7/1953 Arenberg..73/67.8 3,226,975 1/1966 Muller et al. ..73/67.2

OTHER PUBLICATIONS Collier, R. J. et al., Application of MoireTechniques to Holography, Applied Physics Letters, Oct. 15, 1965, Vol.7, No. 8, p. 223-225.

Powell R. L. et al., interferometric Vibration Analysis by WavefrontReconstruction, J.O.S.A., Vol. 55, No. 12, Dec. 1965 p. 1593-1598.

Laser Focus, Holographic Vibration Analysis Promising for NondestructiveUltrasonic Testing, Sept. 1966, p. 29- 3 l.

E. N. Leith et al., Holograms: Their Properties and Uses, S.P.l.E.Journal, Oct./Nov. 1965, pp. 3- 6,

Primary Examiner-Richard C. Queisser Assistant ExaminerJohn P. BeauchampAtt0rneyHauke, Krass & Gifford [5 7] ABSTRACT A transducer inducesvibrations into a member including at least a pair of members joined toone another along a common surface. An exposed section of the twomembers, which may or may not include one line of their joinder, isilluminated with a source of coherent light while it is being vibratedand a photographic plate is exposed to both the reflected light and thedirect coherent light. The plate is developed to provide a hologramwhich is then properly illuminated to recreate a visual image of thesurface section. The visual image will contain fringe lines and/orshaded areas as a result of interference between light waves reflectedfrom the member at the high and low points of its vibration. Anomaliesin these lines and areas will reveal discontinuities in the joinderbetween the two members which affect the pattern of vibration of thesurface section.

8 Claims, 5 Drawing Figures PAIENTEIJFEB29 :912

SHEET 1 OF 3 F-IG- F/ G 2 INVENTOR.

RALPH M. GRANT ATTORNEY PATENTEDrzazs 1972 SHEET 2 OF 3 INVENTOR.

RALPH M. GRANT F-IG- 4 ATTORNE Y mtmmmzs I972 3.645.129

sum 3 BF 3 FIGS INVENTOR RALPH M. GRANT BY i ATTORNEYS METHOD FORANALYZING THE JOINDER BETWEEN A PAIR OF AEUITING MEMBERS CROSS-REFERENCETO RELATED APPLICATION The present application is a continuation-in-partapplication of Ser. No. 633,068, filed Apr. 24, 1967, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a nondestructive test method for determining the adherencebetween a pair of abutting members including inducing vibrations in themembers; creating a hologram of the member as it undergoes vibration;and reconstructing and analyzing a visual image of the members from thehologram, such images including fringe lines and shaded areasrepresenting the modes of vibration of the member.

2. Description of the Prior Art Holography has previously been employedto analyze the vibration of normally vibrating devices such asloudspeaker cones. One of the several available techniques for forming ahologram, i.e., coherent holography or white light holography, have beenapplied to the member, while it is undergoing vibration, to form ahologram. The reconstructed image of the vibrating device contains apattern of fringe lines as a result of the interference between thelight reflected from the object to the photosensitive plate during itsvibratory motion. An analysis of these fringe lines provides informationas to the pattern of vibration of the member and the degree of flexureat any point, which is unobtainable by any other method, be it empiricalor anlytical.

SUMMARY OF THE INVENTION The present invention contemplates an extensionand variation of the above method of analyzing vibratory structureswhich is useful to determine the degree of adhesion between a pair ofmembers which abut one another along a common surface or line. Thesemembers may be intended to be adhered to one another, as in the case ofa welded, brazed or chemically bonded joint, or may be intended to besimply abutting, but not bonded to one another. The members may beformed of the same or dissimilar materials and may be either rigid orelastic. In any of these situations the present method is capable ofdetermining whether a bond exists between the members at all pointsalong their interface and it may also be used to determine the degree ofbond.

In order to practice the method vibrations are induced in the twomembers. In a limited class of cases the members will be part of astructure which normally undergoes vibration, such as the cone and frameof a loudspeaker. In such cases it is simply necessary to induce thevibration of the members by the usual means, such as providing an audiosignal of the proper frequency and intensity to the speaker coil. Inother cases the members will not be part of a normally vibratingstructure and it is necessary to induce vibrations in them.

The method used to induce vibrations in the members is dependent upontheir nature and configuration, and may take the form of contacting themwith a vibrating transducer or coupling them to a vibrating transducerthrough a fluid medium, such as air or water. The frequency andintensity of the vibrations induced in the member are also a function oftheir nature and configuration and may vary over a wide range. Avariable frequency oscillating mechanism might be employed to find afrequency which induces the most active vibrations in the members sincegenerally the most vigorous vibrations would produce the best fringelines in the visual image produced from the hologram. In certainapplications it may be desirable to provide a number of holograms madeof the same structure with inducing vibrations of different frequency oramplitude.

Any of the available methods may be employed to form a hologram ofa'section of the surface of the members while they are undergoingvibration. This surface section may either include part of the boundarybetween the members or will otherwise be sufficiently spatially andstructurally related to their boundary so that anomalies along the jointwill produce at least minute variations in the vibration of the surfacesection. In the practice of a preferred embodiment of the invention,which will subsequently be disclosed in detail, this section isilluminated with a coherent light which is reflected to a photographicplate that also receives direct illumination from the light source.Because of the spatial coherence between the direct and reflected lightthe plate will be exposed to the interference pattern between the twolight sets. The exposure time of the plate is sufficient for the membersto undergo at least one, and preferably a number of vibrations. Thephotographic plate will thus record the interference between the directand reflected light at the two extremes of vibratory motion because ofthe time averaging feature of the photographic emulsion.

When developed, the hologram is appropriately illuminated, normally withcoherent light, and an optical image of the section of the membersholographed will be made visible, and will contain fringe line patternsbased on the movement undergone during the vibrations. The spacing ofthe fringes in each region of the observed section will depend on thedimensions of the vibration which that section underwent and the fringesin particular areas may merge into greyings. The analytic method dependsupon the concept that the vibrations will only be fully propagatedacross the interface at points where the two members are fully bonded toone another.

In certain simple structures where the observed section includes part ofthe interface, a single vibration mode may be set up and the fringelines visible at this point on the boundary will be continuous. Wherethe members are merely abutting one another and not united, independentvibrations will be set up and the fringes at these points will bediscontinuous across the boundary. Completely separate fringe systemsmay be set up on two sides of the boundary, or a fringe shift may benoted at the boundary. No experience is necessary to perform analysis ofthe boundary bond using these principles. After viewing one hologram itmay be necessary to produce additional holograms with varyingfrequencies and intensities of vibrations imposed on the members inorder to produce a fringe system which produces fringes at desiredpoints at which analysis is to be performed.

In cases where the observed section does not include the interface beingstudied, but that interface is rather below the observed section, as inthe case of two rectangular prisms bonded together wherein that bondmust be studied from the surface of one of the prisms, the analyticmethod may be considered to depend on either vibration transmissionacross the interface, or the support of the observed surface by theinterface, or combinations of the two. If the bond between the prisms isinterrupted at a point, the mode of vibration of surface areas proximateto this point will differ from that of other surface areas. Byperforming the analysis over a range of frequencies, it is usuallypossible to find frequencies which excite either the bonded area or theunbonded area, but not both. At these frequencies distinct anomalieswill appear in the fringe pattern of the part of the surface which isrelated to the unbonded area, thus revealing the bond failure.

The subsequent description details a preferred method of practicing theinvention and apparatus suitable for such practice. Various objects,advantages and applications of the invention will be made apparent bythe following detailed description, which makes reference to theaccompanying drawings, in which:

FIG. I is a perspective view of a pair of metal plates which are weldedtogether, illustrating the fringe lines which are visible as a result ofpractice of the method of the present invention;

FIG. 2 is a perspective view of a sonic transducer, illustrating thefringe lines which are made visible by practice of the method of thepresent invention;

FIG. 3 is a schematic view of apparatus arranged to form a hologram of avibrating member;

FIG. 4 is a schematic view of the method of observing the opticalreconstruction of the vibrating member through the hologram; and

FIG. 5 is a perspective view of a member so constructed that the bond tobe analyzed must be studied from a surface which does not contain theinterface, with a fringe pattern shown which is made visible by practiceof the present invention.

Referring to the drawings, FIG. 1 illustrates a pair of plates 7.0 and12 which have been welded together, and illustrates the results achievedupon analyzing that weld through use of the method and apparatus of thepresent invention.

The plate is formed of steel cut into a rectangular shape and has anappreciable thickness. The plate 12 is formed of the same material andhas a 90 bend formed along its length so as to comprise a planar section14 and an extending section 16 which projects normally to the surface14. The plates have been disposed with one edge of the plate 10 inabutment to the lower side of the plate ll2 so as to create a commonsurface l8 and and have been welded along their line of abutment 22.

In order to test the continuity of the weld, the method of the presentinvention, as hereinafter described, has been applied. Upon visualexamination of the optical image of the members 10 and 12 from ahologram formed of the common surface 18 and 20 while the members arevibrated, a plurality of fringe lines 24 appear. If the members 10 and12 are fully welded along the line 22, they will vibrate as an integralplate; otherwise, they will develop independent modes of vibration. Thefringe lines 24 represent nodes of the vibration pattern set up in thesurface 18-20, and a continuity of the fringe lines across the interface22 indicates a good weld while a discontinuity indicates that themembers are vibrating separately and the weld is broken. The fringepattern 24 set up in the surface 18-20 is continuous, except at 26. Thediscontinuity at that point indicates an imperfect weld.

In order to cause the surface 18-20 to vibrate, in the practice of thepresent method, it would be necessary to induce vibrations from someexternal source. Method of accomplishing this will be subsequentlydescribed.

The device of FIG. 2, generally indicated at 30, constitutes anelectrosonic transducer, and it is not necessary to provide an externalvibrating source in order to practice the method of the presentinvention. The transducer includes a vibrating surface plate 32 which issurrounded by a rectangular rubber gasket 34. The gasket is firmlyretained between a rectangular aperture in the housing 36 and the plate32. The vibrating mechanism 38 is supported to the rear of the housing.The inner edge of the rubber gasket 34 is intended to be securely bondedto the outer side of the plate 32 by appropriate adhesives. In order totest the continuity of that bond the cover plate is vibrated; a hologramis made; and the hologram is illuminated and viewed to observe anoptical reconstruction of the plate 32 and the surrounding gasket 34.

Such reconstruction makes visible fringe lines 40. The continuity of thefringe lines 40 in the plate 32 and in the gasket 34 are observed and itis noted that a discontinuity exists at the point 42, indicating animproper bond at that point.

FIG. 3 illustrates the practice of the present invention on a specimengenerally indicated at 50 which consists of a pair of rectangular plates52 and 54, welded together along a line 56. In order to inducevibrations in the specimen, a sonic transducer 58 is positioned to therear of the specimen. The voice coil 60 of the speaker is appropriatelyenergized from a variable frequency audio source 62. The positioning ofthe speaker with respect to the specimen 50 is such as to cause it tovibrate. In certain cases it may be necessary to couple the sonicgenerator and the test specimen with a more dense fluid, such as water.In such cases a waterproof sonic generator would be disposed in thefluid and one surface of the test specimen is immersed in the fluid.Alternatively, it would be possible to use methods of underwaterholography to immerse the entire specimen.

In the preferred embodiment the hologram is formed by illuminating thesurface of the specimen, including the interface 56, with light from alaser source 64. The light is passed through a half-silvered mirror 66,to reflect a portion of the beam directly to the photosensitive surfaceof a photographic plate 68. Light from the laser illuminated surface ofthe test specimen 50 is also reflected to the photosensitive surface ofthe plate 68. The length of exposure of the plate must be sufficient forthe member 50 to undergo at least one, and preferably severalvibrations.

Following the exposure of the plate 68 it is developed in a conventionalmanner to produce a hologram 70 (FIG. 4) which constitutes aphotographic record of the lines of interference between the directlight from the laser 64, via the mirror 54, and light reflected from theilluminated surface to the test specimen 50. The hologram 70 isilluminated by a laser 72 positioned with respect to the hologram as themirror 66 is positioned with respect to the photographic plate 68. Anobserver 74 positioned in the opposite side of the hologram and lookingthrough it will observe a virtual image 76 which is the opticalequivalent of the specimen 50. Not only does it exhibit a stereo deptheffect, but a movement of the observers head produces a shifting of thesurfaces of the image 76 with respect to one another and reveals andhides various surfaces. In addition to containing all the observableoptical properties of the member 50, this image contains a plurality offringe lines 78. Again, the continuity of the fringe lines across theoptical reproduction of the interface constitutes a test of the bondalong the interface.

The method is equally useful where the bond to be analyzed does notoccur at a surface. In such cases the analysis is based on theholographic study of a surface which is so related to the bond thatspecific surface areas may be associated with particular areas of thebond. Such an object is illustrated in FIG. 5 and takes the form of apair of rectangular plates 80 and 82, of similar shape, which are bondedtogether along one of their faces. While a holographic study of one ofthe edges of the assembly which contains the interface between the twomembers might reveal discontinuities in the bond between the members atthe points studied, in order to uncover failures of the bond in areasdeep within the member it is necessary to make a holographic study of asurface such as that denominated as 84 in FIG. 5 which represents thesurface of the member 82 opposite to that which is bonded to the member80. Since the surface 84 lies parallel to the bonded surface, ifvibration is induced in the member 82, from the member 80, through theinterface between the members, the vibration pattern of the member 84will depend upon the condition of the bond between the members.Likewise, if a vibration is induced in member 84 by other means, themember 84 may be considered as being supported by the juncture betweenthe two members so that an imperfect area of the bond will be reflectedin an unusual vibration of the associated area of the surface 84.

In order to test the bond vibrations are induced in the assembly in anappropriate manner and a holographic study of surface 84 is made in thesame manner as previously described. FIG. 5 illustrates a set of fringelines 86 which may result from an incomplete bond between the members 80and 82 at a point on their interface directly beneath the center of thefringe pattern 86.

A fringe pattern such as 86 occurs when the center of the fringe patternvibrates but the rest of the surface 84 does not. This may occur whenvibrations of a frequency which will excite any improperly bonded areas,but not properly bonded areas, is induced in the member. Various methodsof analysis, such as real time holography, or simply formation ofholograms at a number of spaced frequencies may be used to determinefrequencies which will excite a discontinuous section ofu particularmember. Afterwards, vibrations of this par ticular frequency may beinduced in similar members, and holograms made in accordance with the[)I'UCCllli of the present invention, in order to nondestructivelyanalyze the bonds in such members.

It is apparent that the method of the present invention may be readilyused to nondestructively analyze the presence or absence of bond in awide variety of structures. The scope of the inventive method andapparatus should be considered to be limited only in accordance with thefollowing claims:

1 claim:

1. The method of analyzing the bond between a pair of members which abutone another at a joinder, comprising; inducing vibrations in themembers; forming a hologram of a section of a surface of the memberswhile they are undergoing vibration; and illuminating the hologram andviewing a reconstructed optical image of said surface section of themembers, and identifying nonuniform fringe lines, such image displayingfamilies of fringe lines which are generally uniform in areas associatedwith points of the joinder where the members are bonded to one anotherand are nonuniform in areas associated with other points of the joinder.

2. The method of claim 1 wherein the section of surface of which thehologram is formed includes a line of the joinder of the pair of membersand the families of fringe lines which are displayed on the image arecontinuous across the line of joinder at points where the members arebonded to one another and are discontinuous at other points across theline of joinder.

3. The method of claim 1 wherein one of the pair of abutting membersconstitutes the diaphragm of a sonic transducer and vibrations areinduced in the members by energizing the transducer.

4. The method of claim 1 wherein the members are caused to vibrate bycontacting them with a vibrating transducer.

5. The method of claim 1 wherein the members are caused to vibrate byinducing the vibrations into a fluid medium contacting the members.

6. The method of claim 1 wherein the hologram of the vibrating membersis made by exposing a photographic plate to light from a coherent sourcewhich arrives at the plate directly and by reflection from said surfaceof the members and developing the photographic plate.

7. The method of claim 6 wherein the photographic plate is exposed forperiods sufficient for the members to undergo a plurality of vibrations.

8. The method of claim 6 wherein the hologram is viewed by illuminatingone side of it with a coherent light source and observing through theother side.

HIM/5 ll-H

1. The method of analyzing the bond between a pair of members which abutone another at a joinder, comprising; inducing vibrations in themembers; forming a hologram of a section of a surface of the memberswhile they are undergoing vibration; and illuminating the hologram andviewing a reconstructed optical image of said surface section of themembers, and identifying nonuniform fringe lines, such image displayingfamilies of fringe lines which are generally uniform in areas associatedwith points of the joinder where the members are bonded to one anotherand are nonuniform in areas associated with other points of the joinder.2. The method of claim 1 wherein the section of surface of which thehologram is formed includes a line of the joinder of the pair of membersand the families of fringe lines which are displayed on the image arecontinuous across the line of joinder at points where the members arebonded to one another and are discontinuous at other points across theline of joinDer.
 3. The method of claim 1 wherein one of the pair ofabutting members constitutes the diaphragm of a sonic transducer andvibrations are induced in the members by energizing the transducer. 4.The method of claim 1 wherein the members are caused to vibrate bycontacting them with a vibrating transducer.
 5. The method of claim 1wherein the members are caused to vibrate by inducing the vibrationsinto a fluid medium contacting the members.
 6. The method of claim 1wherein the hologram of the vibrating members is made by exposing aphotographic plate to light from a coherent source which arrives at theplate directly and by reflection from said surface of the members anddeveloping the photographic plate.
 7. The method of claim 6 wherein thephotographic plate is exposed for periods sufficient for the members toundergo a plurality of vibrations.
 8. The method of claim 6 wherein thehologram is viewed by illuminating one side of it with a coherent lightsource and observing through the other side.